Photomultiplier tube



y 1963 A. F. MCDONIE ETAL 3,099,754

PHOTOMULTIPLIER TUBE Filed May 5, 1960 INVENTORJ' Arthur F. MDonieRobert M. Matheson United States Patent 3,099,764 I PHOTOMULTIPLIER TUBEArthur F. McDonie and Robert M. Matheson, Lancaster,

Pa., assignors to Radio Corporation of America, a corporation ofDelaware Filed May 5, 1960-, Ser. No. 27,066 1 Claim. (Cl. 313-95) Thisinvention relates to photoemissive devices and has special reference toimprovements in the electron multiplier section of a high vacuum typephotomultiplier tube.

Photomultiplier tubes, having a large area photoemissive cathode surfacethat is exposed to a light source, have been used as scintillationcounters. In the operation of such a tube, when radiations, such asthose from radioactive matreials, are caused to fall upon a phosphor andactivate the phosphor to luminescence, the light from the phosphorcauses photoemission of electrons from the photocathode of the tube. Thephotoelectrons are then directed, by an electrostatic electron-opticalfield, to an electron multiplier structure within the tube. The electronmultiplier structure usually comprises a staggered array of electrodeseach adapted to produce secondary electron emission when subjected toelectron bombardment. Amplification of the current represented by thephotoelectrons from the cathode is thus achieved by secondary electronemission phenomena.

The photomultiplier tubes known in the prior art have been limited inthe magnitude of the output current that can be obtained by themagnitude of the potentials which could be applied to the various stagesof electron multiplication. The reason for the limitation on themagnitude of potentials that could be applied to the electronmultiplying electrodes is that certain dark currents occur within thetube, and the magnitude of these dark currents generally increases asthe magnitude of the potential that is applied to the electronmultiplier electrodes, or dynodes, is increased. The specific causes ofthese regenerative dark currents are not completely understood. One ofthe known origins of these dark currents is that light is generated byelectrons strikinga dynode surface 'at a high velocity. The light thencouples back to the photocathode which produces a spuriousphotoemission. Other reasons for these dark currents include ionfeedback from the dynode structure to the photocathode, meta stable atomor molecule feed-back from the dynode to the photocathode, and X-rayfeed-back from the dynode structure to the cathode. The problem of ionfeed-back occurs generally in the area between the photocathode and thefirst dynode since the potentials are normally arranged so that the ionsare collected by a dynode structure if the ion is originated in anyother area of the device. Also, ions generated in the anode region ofthe multiplier sometimes strike the glass bulb with sufficient energy tocause fluorescence or luminescence of the glass bulb. The light sogenerated may feed back to the photocathode to cause spuriousphotoemission. The meta stable atom or molecule, is a particle having azero electrical charge, can cause photoemission from the photocathode ifit originates in any area of the tube since it is not attracted to andabsorbed by a dynode structure. Also, radiations such as X-rays cancause spurious photoemission from the photo cathode when they originatein any area of the tube since these radiations are not attracted toparticular dynodes.

In the prior art many attempts have been made to decrease theregenerative dark current that has been caused by one or combinations ofthe above mentioned phenomena, so that higher voltages may be applied tothe dynodes and thus, higher output signals obtained. These attemptshave been unsuccessful in that the magnitude of the dynode potentialscan not be substantially increased by using any of these known devices.

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It is therefore an object of this invention to provide an improvedphotomultiplier tube.

It is a further object of this invention to provide a new and noveldynode sub-assembly for use in a photomultiplier tube.

These and other objects are accomplished in accordance with thisinvention by providing an imperforate electrically conductive shieldclosely spaced around the electron multiplier structure.

The invention is described in greater detail in connection with theaccompanying single sheet of drawing wherein:

The single FIGURE is a sectional view of a photomultiplier tube inaccordance with this invention.

The tube shown in FIG. 1 comprises an evacuated envelope 10 closed atone end thereof with a wall section or face plate 12 which extendssubstantially transversely of the tube axis. On the inner surface of thetransparent face plate 12, there is provided a photoemiss-ivephotocathode 14 in the form of an electrically continuous photoemissivesurface. The photoemissive surface may ,be formed of any of the knownmaterials such as, for example, manganeseantimony-oxygen-cesium or amultialkali photoemissive film.

The tube is provided with a cup-shaped dynode shield electrode 16 whichis spaced from the photosurface 1'4 and which terminates in a basemember having an aperture 18 therein. Spaced inside the dynode shield 16is a focus electrode 20 which is a hollow tubular member that is spacedbelow the top of the dynode shield 16 and which includes an aperture 22that is spaced around an inner ring 24. The ring 24 is connected to thedynode shield 16. The inner ring 24 serves to protect the electrodeelements in the tube from contamination by the material which isevaporated from the dynode side of the accelerating electrode or dynodeshield 16, during a step in the manufacture of the photocathode 14.

A metallic wall coating 26, which may, for example, be a relatively thinfilm or layer of aluminum, is formed on the inner surface of the tubeenvelope 10, in the area above the bottom of the dynode shield 16 and byany well known aluminizing techniques. The metallic wall coating 26extends from the photocathode axially down the tube wall and thus formsa cylindrical focusing electrode coaxial with the dynode shield 16.Electrical contact (not shown) is made by means of a lead which connectsthe photocathode through the wall coating 26 to one of the lead in pins28 that is sealed through the base of the tube envelope. The coating 26not only functions as a part of the electron optical field which directselectrons from the photocathode 14 into the photomultiplier structure,but also functions so as to prevent the collection of charges on theenvelope wall adjacent to the photocathode 14.

The photomultiplier structure 30 is of the straight throu-g type andincludes a plurality of elongated electron multiplier electrodes ordynodes 31 through 40. The dynodes 31 through 40 may be made of anyconventional material having a high secondary electron emission, suchas, for example, .a copper beryllium alloy. The dynodes 31 through 40are disposed in a staggered array on opposite sides of a plane so thatphotoelectrons from the photocathode 14 impinge upon the first dynode 31and initiate secondary electron emission therefrom having a ratio thatis greater than unity. This secondary electron emission is acceleratedand directed by fixed electrostatic fields along curved paths tosuccessive dynodes 31 through 40. The accelerating fields are formed bybiasing each successive dynode at a predetermined potential which ispositive, for example, 300 volts positive, with respect to the dynodeprevious to it. Positioned adjacent to the last dynode 40 is a mesh typeanode 42 from which output signals are obtained.

In accordance with this invention the dynodes 31 through 40, as Well asthe anode 42, are enclosed by a substantially imperforate dynode cageshield 44 which extends from the bottom of the [dynode shield 16 aroundall of the dynodes. The spacing between the'dynode cage shield anddynode structure is as close as practical considering the voltageapplied. A spacing of 0.050 is normally used. Tubes having spacingsWithin the approximate range of 0.030 to 0.50 inch have been used. Thedynode cage shield 44 is closely spaced adjacent to each of the dynodesso that an electrical potential is provided in an area that is closelyadjacent to the dynodes and the potential is that of the annular shield16'. The dynode cage shield 44 may be made of any electricallyconductive material such as Nichrome. The dynodes 31 through 40, theanode 42, land the dynode cage shield 44, are supported betweenelectrically insulating dynode spacer sheets 46. One of the dynodespacer sheets 46 has been removed for simplicity of illustration.nection is made to the various dynodes by the support wires passingthrough the dynode spacer sheets to connect to a particular dynode.These wires extend to the lead-in pins 28 passing through the base ofthe envelope '10.

During operation of the device 10, potentials are applied to the variousstructures such as, for example, those shown in the following chart:

It has been found that, due to the presence of the dynode cage shield44, it has been possible to operate ten multiplier stage tubes with upto 4000 volts D.C. applied between the cathode and anode and to increasethe gain of the ten stage photomultiplier as much as ten times thatwhich was possible using the prior art tube structures. In other words,the gain of the tube may be increased as much as ten times thatpreviously known by increasing the dynode potentials by about two timesthat possible in the prior art structures before regenerative Electrodecon- 4 feed-back to the photocathode occurs in an amount equal to thatround in the prior art.

As was previously stated, the reason as to why this invention operatesin the manner described is not clearly understood. Other devices, suchas a mesh screen in the same position as the shield 44, a conductivecoating on the inner walls of the envelope 10, an opaque coating on theouter Wallsof the envelope 10 or an imperforate shield adjacent to thedynode 31 have been tried and have failed to produce any substantialchange in the magnitude of the gain obtainable without increasing thefeed-back action. It is believed that the presence of the dynode cageshield 44 tends to improve the transit time spread, to collect positiveions before they bombard the photocathode, absorb ions which may tend tocause fluorescence on the bulb walls, absorb light generated byelectrons striking the dynode, and collect meta stable particles so thatthese particles, as well as certain generated radiations, do not tend tobe returned to the photocathode to produce the spurious emission.

What is claimed is:

A photomultiplier tube comprising an elongated evacuated envelope, aphotoemissive cathode on the inner surface of one end of said envelope,a tubular cup shaped shield spaced from said cathode and having acentral aperture in the base thereof, a tubular dish shaped focus ringpositioned within said shield and having a central aperture, saidcentral apertures being in alignment, a plurality of elongated electronmultipliers being positioned to receive electrons fromsaid cathodethrough said central apertures, the vertical projection of the aperturein the base of said tubular shield enclosing only the first of saidelectron multipliers, a pair of electrically insulating support memberspositioned adjacent to the ends of said electron multipliers, animperforate multiplier cage shield extending around all of said electronmultipliers and between and supported by said support members, said cageshield being spaced from each of said electron multipliers a distanceless than the length of the shortest of said multipliers, and said cageshield being connected to said tubular shield.

References Cited in the file of this patent UNITED STATES PATENTS

